And in preparation for that, the U.S. is set to test missile defense technology on Tuesday.

The tech's official name is the Ground-Based Midcourse Defense system. When there's an incoming missile, the system shoots its own so the two collide and explode over open water. However, the system's reliability has been the main worry.

In an interview that aired the same day as the missile test, Defense Secretary James Mattis said the U.S. should respond before North Korea develops long-range missiles.

But he also warned if a diplomatic solution isn't found, war with North Korea would be "catastrophic."

Mattis told CBS, "The North Korean regime has hundreds of artillery cannons and rocket launchers in range of one of the most densely populated cities on Earth, which is the capital of South Korea."

For that reason, he said, "Conflict in North Korea ... would probably be the worst kind of fighting in most people's lifetimes."